In continuous polycondensation processes for manufacture of high molecular weight polyester materials, the reactants are generally polymerized in a closed stream under conditions of elevated temperature and reduced pressure. The final step in such polycondensation processes is conducted in a reaction vessel, hereinafter termed a "polymerization finisher" wherein a stream of low viscosity, low molecular weight polyester prepolymer is introduced into one end and, under conditions of elevated temperature and reduced pressure, the stream is removed from the other end as a high viscosity high molecular weight polyester. A polymerization finisher is generally a cylindrical vessel having its longitudinal axis disposed in a substantially horizontal plane and having an inlet for low molecular weight liquid at one end, an outlet for high molecular weight liquid at the other end, and at least one outlet for maintaining reduced pressure in the vessel and withdrawing water and other vaporous by-products from the vessel. Examples of polymerization finishers are disclosed in U.S. Pat. Nos. 3,248,180 and 3,057,702. Polymerization finishers are generally fitted with means for agitating the liquid during the course of the liquid through the finisher; and the outlet for high molecular weight liquid is generally downstream from the agitating means in a pool reservoir for the high molecular weight liquid.
It is well known that processes for manufacturing shaped articles from high molecular weight polyesters result in a large amount of scrap which scrap must be disposed of by some means. For economic reasons, the most satisfactory means for disposing of scrap polyester material is to reuse the material by returning it or its components to the stream of the polyester manufacturing process.
Scrap has been introduced to the stream by being dissolved in one of the monomers upstream from the finisher and before polymerization of the prepolymer has been started. In U.S. Pat. No. 2,933,476 issued Apr. 19, 1960, it is disclosed to add flaked scrap polyethylene terephthalate to the monomer stream in an amount which may be from ten to thirty percent of the total polymer product. Scrap has also been added to the stream by being dissolved in the prepolymer stream at the entrance of the polymerization finisher. Such early addition of polyethylene terephthalate film does permit recycle utilization of scrap material but tends to cause upsets in the process by virtue of the addition of fully polymerized material to monomer or prepolymer streams. Such early addition causes decrease in demand from the monomer supply units. Moreover, additional power is required for early addition of scrap, first to dissolve, and then to move, the polymer-containing stream of increased viscosity.
Scrap has been introduced to the stream by being glycolyzed to break the scrap into monomer and oligomer for repolymerization. In U.S. Pat. No. 3,257,335 issued June 21, 1966, it is disclosed to depolymerize polyester materials and to add the depolymerized material to a polyester manufacturing stream. Such depolymerization provides means for recycling scrap polyester material without appreciable increase in the viscosity of reactant streams; but additional power is required to dissolve, depolymerize, and repolymerize the scrap material. Additionally, scrap recycle by depolymerization causes decrease in demand from the monomer supply and does not provide any increase in the capacity of polymerization equipment. Scrap depolymerized by glycolysis results in formation of ethers, such as diethylene glycol, which ethers become incorporated as ether-linkages in polyester made from the scrap. Polyesters with such ether-linkages present, exhibit undesirable properties as do products made from such polyesters.
Scrap has also been introduced into the polymer stream by melting the scrap and blending it with completely polymerized virgin polymer. U.S. Pat. No. 3,652,466, issued Mar. 28, 1972; Defensive Publication T 870,010, published Jan. 13, 1970; and British Patent No. 1,358,342, published July 3, 1974, all relate to reclaiming polyester by comminuting polyester scrap in accordance with various methods. Each reference discloses that the comminuted scrap can then be blended with completely polymerized virgin polyester material. British Patent No. 1,264,338, published Feb. 23, 1972 discloses that scrap polyester material can be refed along with fresh polyester to a film forming extruder. Such blending of molten scrap polyester with molten virgin polyester requires a separate blending step independent of the polyester polymerization process. Such blending of materials is generally conducted under atmospheric or superatmospheric pressures without occasion for release of dissolved or occluded gases from the scrap material.